China said it has uncovered "irrefutable evidence” of U.S. government cyber attacks on the country’s main agency responsible for timekeeping.

The U.S. National Security Agency has exploited vulnerabilities in some National Time Service Center employees’ mobile phones to attack the devices and acquire sensitive information since March 25, 2022, according to a statement on the official Wechat account of the Ministry of State Security on Sunday.

The U.S. spy agency, the country’s largest to specialize in signals intelligence, has repeatedly used stolen login credentials since April 18, 2023, to hack into the computers at the center, the ministry said.

The NSA didn’t immediately respond to a request for comment sent outside of its working hours.

The accusations follow repeated allegations made by Western governments and companies in recent years blaming Chinese hackers for cyberattacks on their computer systems. They also come as tensions have escalated over trade between the U.S. and China.

China’s Time Service Center, located in the northwestern city of Xi’an, is a vital national facility that provides high-precision service for the government, civil society and the various industries. It also offers important data support for the calculation of international standard time.

China’s national security authorities said investigations showed that private servers all over the world were used to conceal the source of the attacks, the ministry said. Beijing has taken measures to protect against the hacks and has undertaken preventative steps at the time center, according to the statement.